Mac mini has 'Disk Not Ejected Properly' and intermittent audio.

1) The Mac mini's USB port system can only support so many bus powered USB drives and other devices.

Once you go over that power thresh-hold, the Mac mini starts shutting down ports.

2) The un-powered Docking station is only getting power from one of the Mac mini's ports.

Once you go over that power thresh-hold, the Mac mini will shut down that port.

From your description, it sounds like you need to get an additional self-powered hub, plug it into the Mac mini and use it for several of your high demand USB drives and devices.

For some examples of self-powered hubs.

https://www.amazon.com/Rosonway-Powered-Aluminum-Splitter-RSH-H7C4/dp/B0G11F8FHR/ref=sr_1_3?

https://www.amazon.com/UGREEN-Powered-Ports-5xUSB-2xUSB-C/dp/B0GWMGKQSP/ref=sr_1_5?

It could be an issue with your hub not providing enough power for the drives and other components. Is your hub powered or are you relying on the Mac to power? I had a hub under my mini but it had aux power input to help power all the hardware. Also the HDD backup drives may be drawing more power than your Mac can supply. Are those self powered?

If the SSD is ejecting when plugged directly into the Mac mini then I would try another cable and/or ssd enclosure. When testing only have your monitor connected and no other drives or hub hardware. This will verify if the load is too much. Also if you use sleep and your drive is ejecting after the Mac wakes then do not use sleep as this can create all kinds of issues. Just have display turn off after a set time.

"Mac mini has 'Disk Not Ejected Properly' and intermittent audio.: [...]I also have the problem with a 2 other HDD backup drives whether I plug them into the mini or the Anker hub.  Another problem that may or not be related is the connection to my auxiliary speakers.  Sometimes they work and then sometimes they just quit.  I have substituted other cords without success.  The only thing that brings back the sound is to remove the jack from the mini and reinsert it.[...]"

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Single-Out this User:

For troubleshooting purposes, create a Time Machine backup of your Mac, and then create an Administrator user. See if you get the same issue. If not, then it is an issue with your current user. If NOT then it is an issue within your entire Mac. Restore from the Time Machine Backup after the troubleshooting, or log back in to your original user and trash this user you've just created.